1. Field of the Invention
The present invention relates to a fuel cell device, and particularly to a fuel cell device utilizing a fuel cell such as a Direct Methanol Fuel Cell (DMFC), which uses liquid fuel as its fuel.
2. Description of the Related Art
It has been increasingly required to increase lives of cells and batteries in accordance with start of a ubiquitous society. Conventional lithium cells have been developed to an extent close to a theoretical limit, and it is becoming difficult to expect significant improvement of performances. Under such circumstances, attention is being given to fuel cells, which can have significantly increased life owing to its high energy density per weight (volume) as compared with conventional cells.
Among fuel cells, attention has been particularly given to Direct Methanol Fuel Cells (DMFCs), and research has been extensively conducted on such fuel cells because the DMFC has such features that (1) the structure is simple, (2) the fuel can be obtained without large-scale upgrading of infrastructure and (3) it has an inexpensive structure operating at a low temperature, and therefore can be suitably used as fuel cells, e.g., for portable devices such as notebook-size computers, cellular phones and others.
Fuel cell devices employing the DMFC fuel cells can be classified into two types according to the manner of fuel supply. One of the types is referred to as an active type, in which a pump is used for supplying fuel to the cell. The other type is referred to as a passive type, in which a pump is not used, and a capillary force or the like is used for supplying fuel.
Reaction formulas of the DMFC are as follows:
Reaction on fuel electrode side:CH3OH+H2O→CO2+6e−+6H+
Reaction on air electrode side:(3/2)O2+6H++6e−→3H2O
Overall reaction:CH3OH+(3/2)O2→CO2+2H2O
According to the above reaction formulas, equimolar reaction occurs between methanol and water to produce CO2 and two molecules of H2O. However, a low-concentration aqueous methanol solution having a concentration from 3% to 5% is usually used as the fuel, which is actually supplied to a fuel electrode. The purpose of this is to prevent a crossover phenomenon, in which methanol moves to an air electrode after passing through an electrolyte membrane without causing the above reaction on the fuel electrode. The crossover phenomenon occurs more easily with increase in methanol concentration of the fuel. If the crossover phenomenon occurs, the reaction, which must occur on the fuel electrode of the DMFC, also occurs on the air electrode of the DMFC so that the fuel is wasted, and the cell efficiency remarkably lowers due to lowering of the potential on the air electrode side. Accordingly, the low-concentration aqueous methanol solution is usually used.
In the DMFC, a low-concentration aqueous methanol solution is supplied to the fuel electrode as described above. In the active type of the fuel cell device, such a system of a fuel dilution circulation type can be achieved that water produced on the air electrode side and liquid moving from the fuel electrode side are collected, and high-concentration aqueous methanol solution is supplied to the fuel electrode side while diluting it with the liquid thus collected.
The inventors can represent by way of example a generator system shown in FIG. 15. In the system shown in FIG. 15, a load L to be energized is connected to a fuel cell C. A fuel tank t1, a recovery tank t2 and a mixer (mixing tank) MX are arranged outside the cell, and are connected to the fuel cell C via pipes. The water produced on the air electrode side of the fuel cell C and the liquid moving from the fuel electrode side are collected by a pump PM3 into the recovery tank t2. For power generation, a pump PM1 supplies high-concentration aqueous methanol solution from the fuel tank t1 to the mixer MX, and pump PM2 supplies dilution liquid containing water from the recovery tank t2 to the mixer MX. In the mixer MX, the aqueous methanol solution and the dilution liquid are mixed to dilute the high-concentration aqueous methanol solution, and the aqueous methanol solution thus diluted is supplied to the fuel electrode of the cell. A surplus portion of the fuel supplied to the fuel electrode returns into recovery tank t2.
For optimizing the methanol concentration of the fuel supplied to the fuel electrode, the following manner may be employed. A concentration sensor DS detects the methanol concentration of the fuel flowing from the mixer MX to the cell, and a controller CONT controls operations of the pumps PM1 and PM2 based on the value detected by the sensor DS. Thereby, the controller CONT can control a rate in amount between the high-concentration aqueous methanol solution and the dilution liquid to be supplied to the mixer.
The above system can increase the methanol concentration of the fuel in the fuel tank, and can use, e.g., 60 wt % aqueous methanol solution. In this case, the fuel tank can be reduced substantially by a factor from 20 to 12, as compared with the case of using 3 wt %-5 wt % aqueous methanol solution.
Japanese Laid-Open Patent Publication No. 2003-132924 has disclosed another structure, in which a high-concentration aqueous methanol solution is supplied to a dilution tank from a methanol tank via a valve connected to the methanol tank, and water produced by an air electrode of a fuel cell body is collected into a dilution tank. In the dilution tank, the aqueous methanol solution supplied from the methanol tank is diluted with the collected water, and the aqueous methanol solution thus diluted is supplied to a fuel electrode of the cell.
Such a structure is also disclosed that a methanol concentration of the aqueous methanol solution in the dilution tank is detected, and a degree of opening of the valve is controlled based on a result of the detection. Thereby, an amount of the high-concentration aqueous methanol solution supplied from the methanol tank to the dilution tank is controlled so that the methanol concentration of the aqueous methanol solution supplied to the fuel electrode is controlled.
Japanese Laid-Open Patent Publication No. 2003-346846 has disclosed the following structure. A mixing chamber is provided outside a cell body. The mixing chamber is supplied with methanol from a methanol tank, and is also supplied with unused aqueous methanol solution and a by-product from the cell body. A circuit is provided for sending the fuel in the mixing chamber to the cell. The fuel is circulated through this circuit by increasing or decreasing a capacity or volume of a fuel inlet chamber of the cell body or a liquid path communicated with the fuel inlet chamber by a piezoelectric actuator provided for the chamber or the path. In this fuel cell device, high-concentration aqueous methanol solution is kept in the methanol tank. It is considered that the high-concentration aqueous methanol solution can be diluted with water returned from the cell body into the mixing chamber for using the aqueous methanol solution thus diluted as the fuel.
However, the fuel cell device of the active type shown by way of example in FIG. 15 requires three pumps for fuel supply, dilution liquid supply and liquid recovery in addition to the cell body, and further requires a mixing mechanism (mixer) for the high-concentration aqueous methanol solution and the dilution liquid. Therefore, the device has a large and complicated structure, and thus is not suitable for, e.g., a cell device of a portable device.
For suppressing increase in size of the fuel cell device, the device may be of a high-concentration aqueous methanol solution dilution type, which employs a passive-type DMFC not requiring a pump. However, this system structure cannot positively control flow rates of liquids.
Further, the fuel cell devices disclosed in Japanese Laid-Open Patent Publication Nos. 2003-132924 and 2003-346846 require a mixing mechanism (mixing tank) for mixing the high-concentration aqueous methanol solution and the water, in addition to the fuel cell body. This mechanism also increases the device sizes.
Several problems have been discussed primarily in connection with the fuel cell devices of such a type that the DMFC is employed, and the methanol solution is diluted with dilution liquid containing water for supplying the diluted fuel to the fuel electrode of the cell. In general, the fuel cell device of the type, in which liquid fuel is used in the fuel cell, and is diluted with dilution liquid for supplying the diluted liquid fuel to the cell, requires the mixing mechanism (e.g., mixing tank) for mixing the liquid fuel and the dilution liquid in addition to the fuel cell body, and this mixing mechanism increases the sizes of the fuel cell device.